How to Recycle Soil Nutrients? | Soil Management

The following article will guide you about how to recycle soil nutrients.

Introduction:

Farm nutrient balances have become a central tool for assessing and reducing the environmental impact of intensive fanning, they also serve as indicators of sustainable land management. For organic farms in particular an analysis of internal nutrient cycle is essential since purchased fertilizers and fodder have limited nutrient inputs limited. Nutrient management strategies need to be developed to use nutrients efficiently and avoid nutrient losses, ensuring high system productivity.

Nutrient balances are necessary to assess nutrient use efficiency, nutrient turnover and utilization, nutrient enrichment in one part of the farm at the expense of another, and to estimate nutrient losses at farm level. For organic farms, common farm-gate balances yield only global figure because the main nutrient flows take place within the farm.

In organic agriculture, the internal farm nutrient cycle must be quantified to ensure high system productivity accompanied by environmentally sound production processes. In contrast to common farm-gate and field balances, budgeting at the stall level is seldom undertaken. When budgeting mixed farming systems, a substantial lack of nutrients can be detected in the forage and straw input-stall-manure output nutrient flow chain.

Therefore, stall balances focus on a central component of whole-farm nutrient budgets for developing efficient nutrient management strategies. Nutrient balances are very sensitive to variations in mass flow and nutrient content for components with high nutrient contents and/or a large contribution to total mass flow (e.g., manure, silage). In developing strategies to minimize N losses, by reducing N surplus in the ration, one must consider, that, in contrast to dairy farms, a suckler herd for beef production integrated in an organic farm has to adapt to crop production demands.

Nutrients cycle among pools within an ecosystem, and losses of nutrients to the environment accompany each transfer from pool to pool. Efficient recapture of nutrients by plants is critical in extensively managed grasslands if these swards are to persist. In intensively managed systems, the greatest contribution of efficient recapture of nutrients may be minimizing loss of nutrients to the environment and associated negative impacts.

Soil Organic Matter Dynamics and Management:

In a balanced ecosystem, the litter deposition is equal to the litter degradation and soil organic matter is also in equilibrium. Changes in native vegetation, soil-tillage system decrease soil organic matter. The rate of soil organic matter decomposition is associated with greater microbial activity and soil perturbation and lesser residue deposition.

After an initial reduction in the soil organic matter levels (disturbance phase), a new equilibrium is established between litter production and decomposition rates. Soil organic matter stabilizes at lower, the same, or higher levels than the original ones. A considerable amount of N from fertilizer can be incorporated into soil organic matter provided that sufficient residual herbage is left following each grazing. In addition to fertilization, forage use and management may also alter C accumulation.

Recalcitrant compounds are hard to decompose substances and originate either from compounds found in plants (e.g., tannins, lignin, polyphenols), or are formed during the decomposition process. Lignin, C, N, P, and polyphenols and their ratios are often used as indicators of litter quality. As a general rule, legumes have better-quality residue than grasses, and aboveground residues have better quality than roots and rhizomes. However, large variability exists among species.

Thus, the use of plants with low quality residues and higher allocation of biomass to the root system could be proposed as an alternative to increasing soil organic matter. Tropical grasses (e.g., gambagrass, Brachiaria spp.) can increase C storage in the soil not only due to their large root system but also due to the low quality residue originating from this root system.

The organic matter deposited beyond this limit may still undergo biochemical protection by forming recalcitrant compounds; however, this quantity is not well established for particular soils. Unprotected organic matter with higher turnover rates (light fraction) also increases with greater primary productivity, providing more nutrients to the grassland ecosystem.

Improved vegetative cover, land configuration management and alley farming will form the base for sustained production achieving better soil and moisture conservation and improved nutrient recycling.

Plants require 13 essential nutrient elements (6 major, 7 micronutrients) and can profit from some beneficial nutrients. The amount of nutrients required by crops depends largely on the target yield level. Only under rare circumstances the nutrients required for high yields are “automatically” supplied from the soil nutrient reserves.

The response of crop growth and yield to the nutrient supply can be expressed in growth or yield curves and described by “yield laws”. In practical cropping, nutritive minimum factors are often responsible for unsatisfactory yields and thus present growth problems.

Therefore, a proper nutrient management (of the nutrients added to the soil as well as in order to keep many nutrients, especially microelements, on a sufficient natural supply level) is of fundamental importance for effective, successful and sustainable agriculture. Exploitation of soil nutrients is frequently applied which decreases the level of available nutrients quickly or slowly and is therefore non-sustainable.

Incorporation of Plant Residues in Agricultural Systems:

Incorporation of plant residues in agricultural systems is an important factor in the control of soil fertility and maintenance of soil organic matter. If such measures are not taken into account at the right time, then ultimately soil is depleted with deterioration of fertility and productivity side by side. Plant residues are known to affect soil physical properties, availability of nutrients and soil biological activity. However, effects of plant residues on soil and crop differ and depend on their decomposition and nutrient release-rates.

Ground or finely chopped residues material, for example, is likely more susceptible to microbial attack than intact plant parts due to a better soil-residue contact and lack of intact lignified barrier tissues. In contrast to this, however, fine particles are also more likely to be protected against decomposition through physical protection by clay and other particles. The rate of CO₂ efflux, under conditions where moisture and temperature are not limiting, can provide an indication of organic matter quality and whether the soil environment is conducive to the decomposition process.

Carbon Nitrogen Ratio:

Plant residues with high C : N ratio and high lignin and polyphenol contents decompose and release nutrients slowly. Such residues have a low direct nutrient effect and a high indirect mulching effect on crop. In turn, residues with low C : N ratio, and lignin and polyphenol contents decompose rapidly, and have a high direct nutrient effect and a low indirect mulching effect. Therefore, the decompositions of plant residue is related to their C : N ratio and lignin and phenol contents.

Geographical Nutrient Transfer:

In smaller or greater region there are natural nutrient flows and nutrient transfers by trade, e.g., into cities or even export into other countries. These mainly unilateral transfers are of environmental concern since they are not only the cause of nutrient losses from a farming area (and partly completely lost for production), but also unwanted from water eutrophication point of view.

Natural Transfers (Nutrient Flows in Landscapes):

A steady flow occurs naturally with surface or ground water movement in hilly or mountainous areas as part of the slow natural erosion process even under natural vegetation cover. The annual losses from the hilly pat of the landscape are relatively small, and so re the gains for the low- lying land. In geological periods, however, this transfer has produced many fertile alluvial sols in river basins that now belong to the best agricultural lands.

If this process is accelerated by recent man-made strong soil erosion, it can lead to serious soil fertility decrease in the hilly area and to an excessive input on nutrient (especially N and P) into water. The extent of nutrient transfers in small watershed areas or whole river basins is difficult to determine because of many factors involved (sources not only from agriculture, but also from towns, industry, etc.).

From Nutrient Flows to Cycles:

A main objective is converting unwanted flowers into useful cycles. Recycling of nutrients within the farm area tends to be not uniform. For example, slurry is preferably deposited on fields near a farm because of transport costs. On grassland with cows, the manure excreted is distributed very unevenly. Another example is the spotty distribution of ashes after the burning of straw or of the natural vegetation.

None of the cycles is closed, most of them are leaking. Some losses are unavoidable (and may be considered as natural load, a minimum tribute for primary production), whereas, others are avoidable by adequate nutrient management. There is a widespread opinion that decreasing N-fertilization is the best preventive measure for reducing leaching losses of nitrate.

This holds only true, however, for fertilization systems where N is over-emphasized (unfortunately not a rare practice in intensive fertilization cropping), whereas, even high application of N together with a sufficient supply of other nutrients (i.e., with no yield limiting minimum factor) may result only in still tolerable losses.

Cycling is a very complex system. Although it is not so difficult to design a complex model of nutrient flows, their quantitative assessment will partly have to be based on some more or less uncertain estimates. Therefore, the result can only be approximate. For practical use, cycling models should be restricted to the important nutrient fluxes.

Organic Farming and Waste Recycling:

Organic farming is the backbone of sustainable agriculture. Organic farming mainly depends on organic recycling. Industrial agricultural chemicals like fertilizers, pesticides, herbicides, etc., are not used or used to the minimum extent necessary in this kind of farming.

According to the definition of FAO, organic farming should involve successful management of resources for agriculture to satisfy changing human needs while maintaining or enhancing the quality of the environment and consuming natural resources.

Organic farming may result in comparable performance as conventional agriculture and crops grown with high organic manure application could tolerate the pest and disease attack better. It is an ecologically sound and sustainable way of growing more food.

Harnessing the nutrient energy from biological and industrial wastes is of prime importance for maximizing production. When these wastes are recycled as manure for crop production and are subjected to the degradation and assimilative capacity of soil, pollution of streams and/or rivers receiving these wastes is reduced to a large extent as compared to their direct disposal in water bodies.

Organic recyclable wastes include crop residues, animal wastes, farm/industrial wastes, municipal and sewage wastes. They are valuable sources of plant nutrients and humus. In tropical and sub-tropical soils found in India, there is a general deficiency of organic carbon and plant nutrients due to rapid loss of these components by biodegradation.

To make up for these losses, extensive utilization of organic residues in agriculture is essential. In addition, they also protect the soil from erosion. The manurial value and quality of these wastes could be improved by composting and enriching these organic sources along with inexpensive materials such as rock phosphate.

In India, there is a great potential for utilization of crop residues/straw of some of the major cereals and pulses. Approximate availability of straw is to the tune of 141.2 mt, which contribute about 0.7, 0.84 and 2.1 mt. of N, P₂O₅ and K₂O respectively. Considering that about 50% of these residues are utilized as animal feed, the rest could be very well utilized for recycling of nutrients. Crop residues have wide C : N ratio and when applied directly, nutrients may get immobilized leading to initial adverse effect on crop growth.

Hence, adequate care is required to use the residues after proper composting with efficient microbial inoculants. Inoculation of these wastes with efficient cellulotic microorganisms produces enough simple sugars for growth and multiplication of beneficial microflora like free living N fixers and phosphate solubilizes in the soil which ultimately increases crop yields.

While the incorporation of crop residues e.g. wheat and rice straw, as such or inoculated with fungal species had beneficial effects on crop yields and improvement in physico-chemical properties of soils. Incorporation of legume residues viz., green gram or soybean stalks in rice, cowpea stalks in pearl millet or wheat saves energy input besides improving the soil physical and chemical properties.

The Cyclic System for Nutrient Management:

With a system approach i.e. synergistic integration of more than one component, the management of nutrient is to be done in such away so that there is minimum loss along with addition (in organic form) of the nutrient in the soil, taken out in terms of yield. This makes a cyclic system of nutrient management, which has major role in long term sustainable production.

Minimizing Nutrient Losses:

Important loses of nutrients occur through removal of the harvested products, soil erosion, gaseous losses and leaching.

Factors affecting soil erosion are rainfall intensity and quantity of rainfall, erodability of the soil, length and steepness of slopes and soil cover. Traditional soil conservation projects have been emphasizing the use of physical structures (terraces, etc.) which mainly reduce soil erosion by changing the steepness and length of slopes. Current approaches to soil conservation, however, give more attention to reducing soil erosion by improving soil cover rather than the construction of physical structures.

Changes in Soil Slope:

Reducing soil slope by terracing has been the traditional approach to control soil erosion in many soil conservation projects. Terracing is a very effective way of reducing soil erosion if properly constructed. The major disadvantage is the high labour requirement. The current approach is to give less emphasis to complete terracing and instead promote the use of progressive terracing.

Progressive terracing implies that terraces are formed gradually as sediments are deposited in front of a contour barrier. This will often also tend to increase the fertility of the soil in front of the barriers because the deposited sediments are often rich in nutrients.

The barriers can be made out of stones, earth, crop residues, vegetation strips, trash etc. In the case where the barriers are made out of stones or vegetation, the barriers are semi-permeable allowing water to filter through. They will, therefore, to some extent take care of excess water and thereby avoid complete inundation of the field, however, the best choice of barriers depends on the circumstances.

Vegetation strips can also be used as an effective barrier especially on low to moderately sloping land. The simple way to establish vegetation strips is to leave some strips of the land unploughed.

In order arise an interest among the farmers to plant vegetation belts; the plans should have some economic value by, for example, providing thatching material. Farmers should be encouraged to identify local plants, which can be used in barriers.

Species used as vegetation strips should not be too aggressive as they can then easily turn into weeds. If grazing can be controlled, use of palatable species can be used in the vegetation strips. This will offer a much greater range of species to choose from. The barriers should contain species which have a high economic value.

Another simple way to form barriers is to crop residues, weeds or any other material, which is readily available (trash-line). Such barriers are not very suitable on steep slopes as they easily break. The major advantage is, however, that they are easy to make and repair.

Soil Cover:

Improving soil cover can be just as effective in controlling soil erosion as changing the slope. The initial process in water erosion is detachment of soil particles by the force of raindrops. Improving soil cover will reduce the velocity of the raindrops, thereby halting soil erosion in its initial phase. Soil cover and be altered by changing the crop, increasing the yield by use of soil fertility enhancing methods and use of surface mulch.

Controlling soil erosion by changing species composition from annual to perennial crops is an important way of controlling soil erosion. Annual crops are especially inefficient in controlling soil erosion early in the growing season before they become well established.

Perennial plants will cover the ground for a longer period of time and have a more dense cover. Farmers should therefore be stimulated to use perennials such as coffee, tea and fruit trees. It has, however, not been very easy to convince farmers to plant trees in their farm land.

Harnessing Untapped Nutrient Sources:

If used appropriately, the recycling of organic waste from urban to rural areas is a potential, largely untapped, source of nutrients for farm and crop needs, especially on agricultural lands near urban centers. For example, environmentally undesirable wastewater has been used to irrigate fields and return nutrients and organic matter to the soil. Like organic manure, urban waste sludge is a source of primary nutrients, albeit a relatively poor source in comparison with commercial fertilizers.

Stabilized municipal waste sludge typically contains about 3.3 per cent nitrogen, 2.3 per cent phosphorus, and 0.3 per cent potassium, although some concentrations can reach as high as 10 per cent nitrogen and 8 per cent phosphorus on a dry weight basis.

Urban waste also contains organic compounds such as dyes, inks, pesticides, and solvents that are often found in commercial and industrial sludge. These pathogens have been shown to cause genetic damage, while others, such as bacteria, protozoa, and viruses can cause salmonellosis, amoebic dysentery, and infectious hepatitis.

Exploring Internal Nutrient Sources:

Although new sources of nutrients can be developed, genetic engineering offers the potential for plants themselves to generate some of the nutrients they require through nitrogen fixation. In this process, rhizobium bacteria infect, invade, and draw energy from leguminous plants, and in return the bacteria convert and store atmospheric nitrogen in a form that the plant can use for growth.

Besides helping the plants themselves, cereals grown in rotation with leguminous plants can absorb the nitrates released from the decaying roots and nodules of the leguminous plants. Experiments have shown that rice-legume rotations can result in a 30 per cent reduction in chemical fertilizer use.

Better Use of Nutrient Sources:

i. Soil Nutrient and Soil Fertility Aspects:

Soil nutrients are the basic source of the farm nutrient supply. Part of them is utilized by crops, i.e., the easily available portion (water-soluble, exchangeable) as well as the easily mobilizable fraction. The mobilization of nutrients from (mineral and organic) slowly available sources can be enhanced to a certain extent by activating soil life (in general by organic matter of by special biofertilizers), by crop varieties with strongly mobilizing capacity, by better accessibility of nutrients after structure improvement, by deepening of the plough layer or by fallow periods.

The best use of nutrients for crop growth can be obtained on the basis of a high soil fertility level. Soil fertility is a complex term (not very precise, but very useful even in its vague form) which includes many components- soil depth, texture and structure (pore space for supply of oxygen and water), soil reaction, organic matter content and composition, activity of soil organisms, nutrient content, storage capacity for nutrients, content, respectively, absence of detrimental or toxic substances. The result of an optimum combination of these factors is a high soil fertility that means a high crop production potential.

ii. Optimum Soil Reaction:

Many soils, due to a continuous acidification caused by several factors, are a little or much too acidic for high productivity. Therefore, liming is required and the soil should have its optimum soil reaction which varies with soil texture, cropping intensity, etc. The usual (local) recommendations for optimum pH are generally higher with higher clay content and yield level.

iii. Soil Organic Matter:

The qualitative importance of organic matter for soil fertility is rather well known, but many quantitative aspects are still to be solved. The main functions of soil organic matter are physical soil improvement (increase in water and air capacity, protection against erosion, etc.) and several chemical functions (increase of sorption capacity, mobilization of nutrients by mineralization of organic matter and from minerals, short term immobilization into soil organisms (which thus compete with crop for N) and long term fixation into stable humid substances (e.g., improvement of quality via C/N ratio), supply of beneficial organic substances (growth promoters, antibiotics, etc.) The importance of soil organic matter differs in climatic regions, the role of nutrient source often being a dominant one.

In areas of intensive cropping, an increased level and better quality of soil organic matter has significantly contributed to the high productivity level.

iv. Organic Materials:

There is a large variety of materials produced on the farm as well as “imported” from outside, especially communal and food processing waste materials.

Crop Residues and Farm Manures:

Crop residues vary considerably in their nutrient contents (e.g., grain straw vs. green leaves). Residues and manures should be returned to the fields with only little nutrient losses. As for animal manures, the change from the old-type animal manure/straw mixture to slurry (induced by the labour-savings but otherwise somewhat doubtful progress due to straw-less stubbles) has substantially contributed to N-losses.

Main losses of animal sludge distribution occur from ammonia volatilization (sometimes more than half), but losses can be reduced substantially by application of the sludge into the soil instead of spreading it on the surface, the latter procedure being unfortunately still widely used in practice.

In many regions, there is a competitive use of organic matter that could be used as feedstuff, fertilizer, fuel for direct burning or for biogas. As a rule, organic matter fit for soil application should not be burned, although in some situations, there may be little choice for a farmer.

Organic materials should be applied to crops which make the best use of them, e.g., the immediately available ammonia of slurry given to crops before the period of rapid growth, or slowly acting N-sources to crops with a long growing season and with extra profit from the structural improvement for root growth.

Under humid conditions, crop residues are generally mixed into the topsoil as a nutrient source and also for soil improvement. Input of organic material should not only increase, but also improve (higher C/N ratio, etc.) soil organic matter. In arid regions (although an increase in soil humus would even be more important), crop residues are often left on or near the surface for soil erosion control and for water conservation.

Under high yield conditions it may be difficult to get the straw of the previous crop decomposed in a short time and there can be negative effects like temporary fixing of available N (due to the large requirements of the decomposting bacteria). The practice of burning a surplus of straw and just using the ash as a mineral fertilizer is now forbidden in some countries for environmental reasons (smog).

Commercial and Industrial Waste Products:

The waste products that can be used as nutrient sources are materials produced by composting from sewage sludge or (organic) garbage. It is in the interest of towns and food processing industries to return their waste products to farmers’ fields. These waste materials may even be offered as cheap nutrient sources which can save mineral fertilizer input. The advantage for a farmer can be estimated by taking into account the nutrient content and some additional organic matter effect.

There is a growing concern, however, about some toxic elements and even toxic organic substances that might decrease soil fertility and quality of food and fodder products.

Other Organic Fertilizers:

It is well known, that with some biofertilizers the fixing of atmosphere N can be increased by organisms such as Rhizobium, Azotobacter, blue green algae, and Azolla/Anabaena.

Better Nutrient Management for Crops and Crop Rotations:

Adaptation of Crops and Crop Rotations to Nutrient Supply:

For cropping conditions of low natural nutrient supply and restricted input special emphasis must be placed on the nutrient efficiency of crops, mainly uptake efficiency, but possibly also internal use efficiency.

Crop species and even varieties differ to a certain extent in their ability for mobilization and uptake of soil nutrients, especially under conditions of low natural supply. Since under low input conditions the contribution of soil nutrient reserves (especially P and K) is of eminent importance, varieties should be chosen that provide special mechanisms like root excretions for their mobilization and for improved uptake. The same holds true for cases of rather immobile single micronutrients (e.g., Fe, Mn, Zn, Cu deficiency).

In intensive agriculture more emphasis is placed on high yield performance and disease resistance than on nutrient efficiency since it is more economical to supply extra nutrients (for micronutrients even by leaf spray).

The uptake from only slowly available soil reserves can be further improved by choice of crops with an extra potential of nutrient mobilization via mycorrhiza or rhizosphere associated organisms. Symbiotic mycorrhiza fungi (VAM = vesicular-arbuscular mycorrhiza) are widely present in fertile soils (or may be inoculated for special crops).

The effect of this symbiosis is an improved nutrient uptake in general, mainly a better P-uptake of the roots due to better mobilization of soil P reserves and also to more efficient use of added P fertilizers, furthermore an improved uptake to other nutrients with low mobility like some micronutrient metals. VAM improves the productivity by reducing the input, whereas relying more on soil nutrient reserves, but also by enhancing plant survival under nutritional stress conditions.

The most striking example of crop adaptation to nutrient deficiency is the cropping of legumes with rhizobia for fixing nitrogen. An example typical for paddy rice is the Azolla-Anabaena symbiosis.

Not only a crop has to be adapted to nutrient supply, but also general agronomic measures like proper weed control (saving nutrients for crops) and efficient crop protection (for obtaining and protecting high yields) are required to make the best use of nutrient sources.

Adaptation of Crop Rotations to Nutrient Supply:

Although monocultures (like tree plantations or sugar cane) offer the easiest way of production on arable land, many crops are not self-compatible and a farmer usually has to consider the nutrient supply of crop rotations (cropping systems).

Rotations are planned for several (partly competing) reasons: for special products wanted due to economical reasons, for maintenance of soil fertility, for plant disease and pest control, for nutrient supply, etc.

From the crop nutrition point of view, different aspects have to be considered. Rotations can consist of crops with high or low nutrient requirements or of crops known either to improve or decrease soil structure and activity of soil life.

The well-known example of legumes alternating with non N-fixing crops should also be mentioned. Rotations may be chosen which do not leave the sol without green cover (intermediate crops, green manure or even weeds) in order to prevent nutrient losses form bare fallow in the period with pronounced leaching.

The soil nutrient supply may also be improved by not using one crop, but crop combinations like intercropping or mixed cropping. A widely applied practice is the simultaneous growth of grasses with legumes in fodder production.